How can I resize/move the "canvas" of an image in the compositor?
There was a bug report on this where Ton said:


  Blender compositor is an extension of our render pipeline, animation frames by definition are always the same size.
  
  This is an issue we like to work on though, it's called "Canvas awareness" for compositor, where you can define a different output size for it than renders, and transform the renders inside the canvas freely.


There's currently two options:


Perform transforms in 3D and used to resulting render layer. The planes from images addon can help with the setup.
Perform a composite operation against an image of the desired size. The easiest way to generate one, is using the Mask node with a Fixed size.